Where Does Nitrogen Oxide (NOx) Come From?
From the three formation routes to what is specific to incinerators, and how combustion controls differ from post-combustion treatment
01 · DEFINITION
What exactly does NOx mean?
Nitrogen oxides (NOx) is the collective term for gaseous compounds of nitrogen and oxygen. They form during combustion, and the term mainly refers to nitric oxide (NO) and nitrogen dioxide (NO₂). The U.S. Environmental Protection Agency (EPA) uses NO₂ as the indicator for the NOx group as a whole.[3]
Most of the NOx in flue gas leaving the stack is NO. EPA data put NO at 95% or more by volume of the NOx in combustion flue gas.[1] After release, NO oxidizes to NO₂ in the atmosphere, so the NO₂ share rises in ambient air.
| Type | In flue gas and ambient air | Role in regulation |
|---|---|---|
Nitric oxide (NO) | Most of the NOx in flue gas[1] | Expressed as NO₂ and added to the NOx concentration |
Nitrogen dioxide (NO₂) | Small amounts in flue gas; increases in air as NO oxidizes. Irritates the airways and contributes to ozone and fine particle formation[3] | Indicator for the NOx group as a whole[3] |
Nitrous oxide (N₂O) | Small amounts, a few ppm, in coal combustion data[1] | Classified as a greenhouse gas and treated separately from NO and NO₂[4] |
In the molecular models (concept drawings), blue spheres are nitrogen (N) and red spheres are oxygen (O).
When reading an emission limit, check its basis first. In Korea, Annex 8 of the Clean Air Conservation Act Enforcement Rule sets NOx as a concentration (ppm) expressed as NO₂, and for incineration facilities the value is corrected to a standard oxygen concentration of 12% O₂.[5] In other words, the analyzer reading is not compared with the limit as is; the corrected value is.
No. Under Korea's rule, the limit is a NOx concentration expressed as NO₂.[5] Before comparing, check whether the analyzer reads NO only or NO plus NO₂, and whether the NO₂ conversion and the standard O₂ correction have been applied.
Figure 1. Incinerator furnace cross-section showing nitrogen entering with combustion air and with the waste (concept)
Blue spheres are nitrogen (N), red are oxygen (O), gray are carbon (C) and white are hydrogen (H). Nitrogen in air appears as N₂ molecules, and nitrogen in waste as compounds bonded to carbon and hydrogen (conceptual). The blue flows under the grate and along the walls show combustion air being supplied, not SNCR reagent injection.
Source: Based on Micro-one in-house technical data · AI-assisted 3D concept illustration
02 · FORMATION
How does NOx form during combustion?
NOx formation falls into three routes depending on where the nitrogen comes from. Nitrogen in air reacting at high temperature gives thermal NOx, nitrogen bound in the fuel gives fuel NOx, and reactions early in the flame give prompt NOx.[6]
Fuel NOx: comes from nitrogen in the fuel
Forms when nitrogen compounds bound in the fuel or waste react with oxygen.[6]
Prompt NOx: small amounts form early in the flame
Forms early in the flame when hydrocarbon radicals from the fuel react with nitrogen in air.[6]
Figure 2. Conceptual comparison of thermal, fuel and prompt NOx formation routes
Source: Based on Micro-one in-house technical data · AI-assisted 3D concept illustration
The three routes respond to temperature differently. So one change in combustion conditions can cut NOx from one route while leaving another unchanged. The graph below shows only the direction in which each route's formation changes as combustion temperature rises.
Figure 3. NOx formation trends with combustion temperature (concept). The graph illustrates trends only and does not show measured data.
Source: Based on EPA AP-42[1] and NPTEL lecture notes[7] · Prepared by Micro-one
- Thermal NOx: rises steeply as temperature increases.[1] Lecture notes describe it as small below about 1,300°C.[7]
- Fuel NOx: little temperature effect, so the line is nearly flat.[7] Its size depends on the nitrogen content of the fuel or waste.[1]
- Prompt NOx: rises gradually with temperature.[7]
In short, controlling combustion temperature works well on thermal NOx, while fuel NOx depends more on the nitrogen content of the waste than on temperature. The vertical axis shows a relative trend for each route, so the lines cannot be compared with each other in size.
03 · INCINERATOR
Which NOx route is larger in an incinerator?
In incinerators, fuel NOx from the nitrogen in the waste tends to make up a large share. U.S. EPA data relate 70–80% of the NOx formed in municipal waste combustors (MWCs) to nitrogen in the waste.[2] The same source notes that nitrogen in waste converts to NOx even at relatively low temperatures (below 1,090°C).[2] The rest comes from nitrogen in air reacting at high temperature.[2]
These figures are for U.S. municipal waste combustors. In industrial or medical waste incinerators, or at individual plants, the share can differ with waste composition and combustion method.
When the waste changes, NOx moves with it
Fuel NOx depends on the nitrogen content of the waste. So in general, when the composition of incoming waste changes, the NOx concentration can change too. Comparing operating records with the waste delivery history helps narrow down the cause of NOx swings.
Combustion conditions are not set by NOx alone
An incinerator burns waste completely by controlling temperature, oxygen (excess air ratio) and residence time. Lowering combustion temperature or oxygen to cut NOx can increase carbon monoxide (CO) and unburned matter. NOx reduction should therefore be evaluated while checking combustion conditions.
04 · IMPACT
Why is NOx regulated?
Because NOx reacts with other substances in the air to form ozone and fine particulate matter. EPA explains that NO₂, together with other NOx, reacts with chemicals in the air to form particulate matter and ozone.[3] It also reacts with water and oxygen to form acid rain.[3]
In people, high concentrations of NO₂ irritate the airways. People with asthma, children and older adults are especially vulnerable.[3] EPA names fuel combustion as the main source of NO₂ and lists power plants among the emitters.[3] Incinerators, and cement, steel, refining and chemical plants that burn fuel or raw materials at high temperature, are sources in the same category.
In Korea, NOx emission limits are set separately by facility type, size and installation date.[5] The limit values and regulatory structure will be covered in a later column comparing SNCR and SCR.
05 · CONTROL
How do combustion controls differ from post-combustion treatment?
The main difference is when they act. Combustion controls cut formation by changing the conditions under which NOx forms, while post-combustion treatment reduces NOx already formed in the flue gas.
Combustion controls include reducing excess air, staged combustion that splits the air supply, flue gas recirculation (FGR) and low-NOx burners. In FGR, recirculated flue gas dilutes the flame, lowering its temperature and suppressing thermal NOx.[6] The main post-combustion methods are SNCR (selective non-catalytic reduction) and SCR (selective catalytic reduction). Both use a reagent such as ammonia or urea to convert NOx to nitrogen.
Figure 4. Where combustion controls, SNCR and SCR act (concept). Actual plant size, temperatures and layout differ.
Source: Based on Micro-one in-house technical data · AI-assisted 3D concept illustration
No. It is an example that shows SNCR together with SCR downstream of the dust collector to explain where each acts. Actual plant configuration, and whether methods are combined, depends on site conditions. SNCR may be applied inside the furnace or near its outlet, but functionally it is a post-combustion technology because it reduces NOx that has already formed.
| Item | Combustion controls | Post-combustion: SNCR | Post-combustion: SCR |
|---|---|---|---|
| Where it acts | Inside the furnace, where NOx forms | NOx already formed, in the hot zone inside the incinerator | Flue gas downstream of the combustion unit |
| Method | Adjusting excess air, air staging and temperature; FGR; low-NOx burners | Ammonia-based reagent injected into the hot zone without a catalyst[8] | Reagent and a catalyst bed convert NOx to nitrogen and water[9] |
| Operating temperature | Depends on combustion conditions | Optimum about 870–1,100°C for ammonia[8] | Typically about 250–427°C for the catalyst[9] |
| Reduction in incinerator data | Low to moderate in MWCs[2] | 45% in U.S. MWC data[2] | Up to 80% observed; can vary with catalyst poisoning and deactivation[2] |
| Watch points | Limited effect in incinerators with a large fuel NOx share; check combustion (CO, unburned matter) as well | Efficiency can drop outside the temperature window, and unreacted ammonia (ammonia slip)[8] must be controlled | Needs the catalyst's active temperature; ammonia slip can rise as catalyst activity declines[9] |
Are combustion improvements alone enough in an incinerator?
Many reports indicate that combustion improvements alone rarely cut incinerator NOx by much. EPA data note that because the fuel NOx share is large, combustion controls give only low to moderate reductions.[2] That does not make combustion improvements unnecessary. Coal boiler data also show a higher reduction range for low-NOx burners combined with SNCR (50–80%) than for low-NOx burners alone (35–55%) or SNCR alone (30–60%).[1] The two are often used together rather than as alternatives.
For post-combustion treatment, the temperature location is key
SNCR needs a hot zone where the reagent can react (about 870–1,100°C for ammonia).[8] SCR typically needs a catalyst reaction temperature of about 250–427°C.[9] In incinerators where the flue gas after the dust collector is cooler than this, a conventional SCR may need reheating. Whether to place SCR upstream or downstream of the baghouse is covered in How Is an Incinerator Flue Gas Treatment System Arranged?, and a comparison of the efficiency and cost of the two methods will follow in a later column.
06 · CHECKPOINTS
In what order should a NOx problem be checked?
Read the current state before choosing a reduction method. Steps 1–3 read the current state, and steps 4–6 choose the approach.
- Check how readings are expressed: Confirm the NO vs. NOx distinction, the NO₂ conversion and the standard O₂ correction (12% for incineration facilities under Korea's rule).[5]
- Check concentration swings: Use operating records to see how much NOx moves with waste composition and load.
- Check combustion conditions: Confirm that furnace temperature, oxygen (excess air ratio), residence time and CO are within the design range.
- Check room for combustion improvement: Assess how much air supply or recirculation adjustments can cut NOx, together with combustion stability.
- Check where post-combustion treatment can fit: On the plant layout, find the hot zone available for SNCR (about 870–1,100°C for ammonia)[8] and a location that provides the SCR catalyst reaction temperature.
- Check downstream conditions: Baghouse inlet temperature, dust properties and acid gas conditions should suit the post-combustion method. Differences between dust collection methods are covered in the column Electrostatic Precipitator (ESP) vs. Baghouse: What Is the Difference?.
FAQ
Frequently asked questions
Are both NO and NO₂ counted as NOx?
Yes, both count as NOx, but they are not the same substance. NOx in flue gas is mostly NO,[1] which turns into NO₂ in the air. Korea's emission limits are set as values expressed as NO₂.[5]
Do incinerators also form a lot of thermal NOx?
Not mainly. In U.S. municipal waste combustor data, 70–80% of NOx was related to nitrogen in the waste.[2] The rest comes from nitrogen in air reacting. The share can vary with waste composition and combustion method.
Does lowering combustion temperature always reduce NOx?
No. A lower temperature cuts thermal NOx, but fuel NOx is little affected by temperature.[7] Because fuel NOx makes up a large share in incinerators, combustion improvements alone are reported to give low to moderate reductions.[2] Lower temperatures can also make complete combustion harder, so combustion conditions must be checked too.
- U.S. EPA, "AP-42 Section 1.1 Bituminous and Subbituminous Coal Combustion"
- U.S. EPA, "AP-42 Chapter 2.1 Refuse Combustion"
- U.S. EPA, "Basic Information about NO2"
- U.S. EPA, "Overview of Greenhouse Gases"
- Korea Ministry of Government Legislation, Korea's Clean Air Conservation Act Enforcement Rule, Annex 8, "Permissible Emission Standards for Air Pollutants" (in Korean)
- U.S. EPA, "AP-42 Section 1.4 Natural Gas Combustion"
- NPTEL, "Module 7: Combustion and Environment" (lecture notes)
- U.S. EPA, "Air Pollution Control Cost Manual 7th Edition, Chapter 1 Selective Noncatalytic Reduction", 2019
- U.S. EPA, "Air Pollution Control Technology Fact Sheet: Selective Catalytic Reduction (SCR)"